This invention relates to a control device for a vehicle AC generator.
One example of a conventional control device for a vehicle AC generator is as shown in FIG. 5. In the case of the control device shown in FIG. 5, an AC generator 1 has an armature coil 101 and a field coil 102. A rectifier 2 for rectifying AC output voltages is connected to the AC generator 1. The rectifier 2 has a main output terminal 201, an auxiliary output terminal 202, and a ground terminal 203. One end of the field coil 102 is connected to a voltage regulator 3. The voltage regulator 3 comprises: voltage division resistors 301 and 302 for voltage detection; a control transistor 304 whose base electrode is connected through a Zener diode 303 to the connecting point of the two voltage division resistors 301 and 302; a power transistor 305 which is controlled by the transistor 304; a resistor 306; and a suppression diode 307.
The main output terminal 201 of the rectifier 2 is connected to a battery 4. The other end of the field coil 102 is connected through an initial exciting resistor 6 and a key switch 5 to the positive terminal of the battery 4.
The positive terminal of the battery 4 is connected through a load switch 8 to a vehicle electrical load 7.
The conventional control device thus designed operates as follows: When the key switch 5 is turned on, an initial exciting current flows in the field coil 102 through the initial exciting resistor 6, so that the field coil is placed in externally excited state at first. When field current flows and the generator 1 is driven, a power generation starts to induce AC voltage across the armature coil 101. And the field coil 102 is excited by the voltage provided through the auxiliary output terminal 202 of the generator 1, so that the former is placed in self-excited state.
The AC voltage induced on the armature coil 101 is rectified by the rectifier 2 and applied to the battery 4 to charge the latter 4. If, in this operation, the output voltage of the generator 1 is lower than a predetermined value, the voltage provided by the voltage division resistors 301 and 302 is also low. Accordingly, the Zener diode 303 is non-conductive, and the control transistor 304 is also non-conductive, so that the power transistor is conductive. As a result, the field current flowing in the field coil 102 is increased, and the output voltage of the generator 1 is increased accordingly. When, on the other hand, the output voltage of the generator 1 is higher than the predetermined value, the voltage provided by the voltage division resistors 301 and 302 is also high. Hence, the Zener diode is rendered conductive, and the control transistor 304 is also rendered conductive, so that the power transistor 305 is rendered non-conductive. Therefore, the field current is decreased, and the output voltage of the generator 1 is decreased accordingly. The above-described operations are repeatedly carried out until the output voltage of the generator 1 reaches the predetermined value. The suppression diode 307 is to absorb the surge induced on the field coil 102.
In this case, the maximum output of the generator depends on its speed of rotation as indicated by a characteristic curve "a" (broken lines) in FIG. 4; and the output is saturated with the speeds of rotation of about 5000 rpm and higher. The cross point of the generator output to the total vehicle electric load indicated by a characteristic curve "d" (two-dot chain line) is generally set at about 2500 rpm.
With the above-described conventional control device, upon start of power generation the field coil is excited by the output voltage of the generator 1; that is, it is self-excited. As was described above, the maximum output of the generator provides the characteristic curve "a" in FIG. 4, which depends on the speed of rotation of the generator, and the cross point of the generator output to the total vehicle electric load indicated by the characteristic curve "d" is set at about 2500 rpm. Therefore, when the range of speeds of rotation is divided into a high speed range and a low speed range with respect to about 2500 rpm as a border line, if the frequency of operation in the high speed range is in balance with that in the low speed range, or the former is higher than the latter, then no problem arises in view of the balance of charging and discharging the battery. However, if the frequency of operation in the low speed range lower than about 2500 rpm is increased as in the case of a traffic jam, then the output of the generator is liable to become insufficient; that is, the battery is liable to be insufficiently charged. Especially, when the vehicle is run at low speed at night using the head lamps so that the electric load is high, the battery may die to stop the engine.